In the US railroads generally were right hand running, with exceptions as noted above for better grades for the most part.
They were in fact signaled for current of traffic directional running for years. For example, the line I run in the northeast was at least 2 main tracks in its prime. Some locations had more. The line runs east-west. So the eastbound track would be right hand running with all the signals facing you while heading east, the reverse if you were going west.
If you had to run against this current on either track you would not have any signals for that direction. You were issued paper orders to operate against the current of traffic on that track between specific points. You’d be in dark territory because of this and that would bring some limitations that otherwise would not apply.
My line operated this way until the late 1980’s. It was converted to bi-directional signaling after that. After that any train could take any track in either direction, but we still generally run right handed.
Perhaps you’re conflating two different Paris networks.
The Paris Métro is right-hand running. If there’s an exception, it doesn’t immediately occur to me. It’s rare for shallow subways to have separate tunnels for the two tracks, so “switching back and forth” would be quite impractical. On the other hand, France’s mainline railroads (now SNCF) are left-hand running. The RER was created from suburban railway lines, so it is left-hand running, even within the city.
In Madrid, it’s the other way round. The Metro is left-hand running, while mainline and suburban trains (RENFE) are right-hand running.
No, this is definitely the Metro. I’m in Paris regularly, and additionally I have a kid who insists on opening the manual door (pushing the green button or flipping the latch) when it’s available, so I’m always paying attention to the doors. You’re right that it’s predominantly right-hand running, and I don’t want to overstate the notion that the running direction is being reversed willy-nilly, but for certain there are a handful of stations with opposite-side platforms.
I’ve only been in Paris twice, so this is in no way criticism based on superior knowledge, but I have trouble envisioning station layouts that would require the trains to switch right/left tracks. Are you confusing changes in platform orientation with changes in the orientation of track directions?
I mean, I’ve ridden a lot of “drive on the right” metros and whether you get off on the left or the right has depended on whether the tracks split apart for a single central platform (you exit on the left) or stay together with platforms on either side (you exit on the right).
All but a few metros have a combination of side and island platforms, meaning the doors sometimes open on the right and sometimes on the left. Of course, that’s irrelevant to the way the trains run on double track. In Paris, that’s right-handed.
I’m happy to look up track diagrams if you can mention a particular line or station.
If your memory comes from observing the orientation of the doors, then it could be a result of changes in station layout. You can have the “island” layout, where the two platforms are in the middle back-to-back, and the two tracks run on either side of this platform island; in this case, in a right-running system, the doors of the trains open to the left. But you might also have station layouts where the two tracks are in the centre next to each other, and on either side of these tracks there is a platform; in this case, the doors of a right-running train open to the right. In the city where I live, both station layouts exist in the subway system, but the system is consistently right-running.
Just to come back to this, I’ve gone back over my last couple of Paris visits and I can’t recall the lines or stops in question; we use the Metro heavily when we’re in the city so it’d be difficult to narrow anything down. It’s entirely possible I’m misperceiving or misremembering things, though, so as noted I will pay closer attention on my next visit and then add further details.
Here’s a detailed track map of the Paris Métro. (PDF) I see no locations where trains could cross over to run lefthanded. However, I’m also struck by the nearly total lack of island stations (center platforms between the tracks). So doors would “open on the left” at only a couple of places other than terminals, where it’s common practice around the world to pull a train in on the track it will load and depart from a few minutes later.